Photographic printing



D. R. CRAIG 2,969,723

PHOTOGRAPHIC PRINTING Original Filed June 17, 1957 Jan. 31,, 1961 22A.C.INVERSE FEEDBACK x AMPLIFIER X 2O 26 /gzx POTENTIOMETER FE H o r o32 REFERENCE VOLTAGE a Io INTEGRATING \SWITCH I I4 22 3o A I A.C.INVERSEH 28 )I FEEDBACK I2 AMPLIFIER/ 20% 36 I 32 24 II REFE'RENOEJ REFERENCEVOLTAGE voLTLAGE I 6 8 INTEGRATING 1 3 SWITCH I/ I0 SWITCH I4 J I2 A.C.INvERsE FEED BACK 4 AMPLIFIER REFERENGE x F". 2

v0 LTAGE 3/" INVENTOR INTEGRATING DWIN R. CRAIG ,swITcI-I l 7 M BIO IIBy ATTORNEY PHOTOGRAPHIC PRINTING Dwin R. Craig, F all s Church, Va.,assignor to Logetron'ics, Inc., Alexandria, Va., a corporation ofDelaware Original application June 17, 1957, Ser. No. 666,123,

n ow Patent No. 2,960,019, dated Nov. 15, 1960. Diiglggsand thisapplication Apr. 1, 1960, Ser. No.

8 Claims. (CI. 95-73) This invention relates to photographic printingand is concerned with methods and apparatus for producing What is knownas partial dodging.

The invention is related to photographic printing utilizing a cathoderay tube as the light source in conjunction with inverse feedback from aphototube to a control element of the cathode ray tube to achieveautomatic dodging and automatic exposure control of the types set forthin a number of pending applications filed in the name of the presentinventor.

Where infinite inverse feedback is employed in such arrangements, anydensity variations in a negative will be matched precisely by brightnessvariations of the light produced by the cathode ray tube, completelyeliminating any significant variations in brightness sensed by thephototube, and accordingly eliminating any variations in brightness overthe surface of the photosensitive material to be exposed by the lightemanating from the cathode ray tube. Images thus produced are said topossess zero gross contrast which is defined as density differencebetween regions larger in area than the spot projected at thetransparency. Detail contrast on the other hand, is defined as densitydifferences Within regions smaller than the spot projected at thetransparency. Prints having zero gross contrast are usually veryinformative, exhibiting all details which were present in the negativeor other transparency. They are not pleasing in appearance however,lacking the aesthetic appeal ordinarily sought in subjectivephotography. In accordance with the present invention, methods andapparatus have been developed for producing prints which exhibitdesirable amounts of gross contrast by the use of partial dodging.

It is among the objects of this invention to provide a photographicprinting method comp-rising energizing a cathode ray tube to direct alight spot of finite area through a transparency on a photosensitivesurface to be exposed, sensing the intensity of light to which thesurface is exposed, and during an exposure cycle controlling theexposing light impinging on the surface to provide one exposing lightcomponent varying over said surface as an inverse function of the sensedintensity, and another exposing light component substantially equallyilluminating the entire area of the transparency. In accordance with theforms of the invention contemplated, the components of exposing lightmay be provided simultaneously or sequentially and the substantiallyequally illuminating component may be produced optically by diffusion ofa portion of the light from the tube or electrically by rendering thecontrol of the tube independent of the sensed intensity. The proportionsof the components may be controlled by integrating light to which thesurface is exposed, the exposing cycle may be controlled by integratinglight to which the surface is exposed and both the exposing cycle andthe proportions of the components may be controlled by integrating lightto which the surface is exposed.

The invention also contemplates photographic printing apparatuscomprising a cathode ray light source having nited States Patent 6 acathode and control electrode means, support means for a transparencyand a photosensitive surface to be exposed in the optical path of lightproduced by the source, a photoelectric sensing device in the opticalpath of light to which the surface is exposed, an inverse feedbackamplifier in circuit with the sensing device and control electrodemeans, and control means for varying the local effects of inversefeedback of the amplifier on the surface to provide different degrees ofpartial dodging. The control means may include an optical diffuserintermediate the source and transparency, a voltage divider intermediatethe amplifier and control electrode means, and/or exposing lightintegrating means and switching means in circuit with the amplifierresponsive to the integrating means.

A more complete understanding of the invention will follow a descriptionof the accompanying drawing wherein:

Fig. l is a diagrammatic representation of one form of apparatuscontemplated;

Fig. 2 is a diagrammatic representation of a modified form of apparatus;and

Fig. 3 is a diagrammatic representation of a third form of apparatus.

With particular reference to Fig. 1, the light source assumes the formof a cathode ray tube 10 including conventional means for producing ascanning electron beam and consequent scanning spot of emitted light andhaving a cathode l2 and a control electrode 14. Light from the face ofthe cathode ray tube is projected by a lens 16 through a transparency 18on the surface of a photosensitized body 20. Light impinging on thephotosensitive surface is sensed by a phototube 22 whose output isconnected with an AC. inverse feedback amplifier 24 connected in circuitwith the control electrode of the cathode ray tube by means of anattenuator 26 which may assume the form of a potentiometer asdiagrammatically shown in Fig. 1. Light impinging on the surface of thephotosensitive body 20 is also sensed by a phototube 28 whose output isfed to an integrating switch 30 under the control of a reference voltage32 to modify the effect of the control electrode 14. The integratingswitch may assume various forms exemplified by a capacitor on which acharge is built up until it is sulficient to fire a thyratron which willin turn apply such voltage in conjunction with the reference voltage 32to the control electrode 14 of the cathode ray tube to terminate theexposure. By selecting the tap of the potentiometer 26 of Fig. l, theproportion of inverse feedback applied to the control electrode 14 ofthe cathode ray tube can be varied in definite steps or infinitely sothat the light impinging on the photosensitive surface and sensed by thephototube 22 will have a component varying over the surface as aninverse function of the sensed intensity and another componentsubstantially equally illuminating the entire area of the transparency.The phototube 28 is depicted as sampling a local area since its functionis to terminate the exposure after a prescribed quantity of light hasbeen directed on the surface.

Inasmuch as the method and apparatus of Fig. 1 are subject to changes inthe loop gain due to variables such as the lens aperture, averagenegative density, transmission of the material constituting thephotosensitive body and collection efiiciency between the printing stageand phototube, as well as sensitivity of the phototube, it is desirableto provide methods and apparatus which will be independent of suchvariables as will be described with reference to the other figures.

In Fig. 2 the dodging loop between the phototube 22 and controlelectrode 14 contains only the AC. inverse feedback amplifier 26 so thatin this case the overall loop gain can be considered to be infinite sothat if other provision were not made, the image produced on thephotosensitive material would possess zero gross contrast, or in otherwords would involve one hundred percent dodging. In this case, thepartial dodging is achieved by purely optical means by inserting adiffusing transmitter of light 34 between the lens 16 and the trans.-parency 18. Were this diffuser 34 imposed completely in the light path,the source would behave much the same as though it were an ordinary lampsource. Accordingly, the diffuser 34 will be interposed only partiallyin the optical path so that the printing plane at the photosensitivesurface will be illuminated partially by diffused light passing throughthe diffuser and partially by the scanning spot from the cathode raytube which has not passed through the diffuser.

It is interesting to note in this connection that the existence of suchdiffusing light accounts for two distinct but cumulative effects servingto reduce dodging. One of these effects is purely photographic wherebyan undodged component of light is received by the photosensi-. tivesurface. The second effect is the apparent reduction in contrast of thetransparency to which the phototube 22 is exposed. When the scanningspot encounters a dense area in the transparency, it produces a signalwhich causes the cathode ray tube to produce more light, but since thebrightness of light at the diffuser is directly proportional tobrightness of the cathode ray tube, light, passing through the thinareas of the transparency by way of the diffuser reaches the phototube22 to satisfy its demand for additional light. Consequently, the cathoderay tube will never become sufiiciently bright in the dense areas of thetransparency to produce a completely dodged print. The combination ofthese two effects, being additive, account for the fact that only asmall portion of the lens need be covered by the diffusing material toproduce a large reduction in the dodging effect even though the loopgain is for all practical purposes, infinite.

Thus, in conjunction with the form of the invention disclosed in Fig. 2there is also a component varying over the surface as an inversefunction of the sensed intensity and another component substantiallyequally illuminating the entire area of the transparency.

With reference to Fig. 3, the partial dodging is achieved by providingthe separate light components sequentially. The initial portion of theexposing cycle will be effected with one hundred percent inversefeedback, following which there will be an exposure involving nofeedback whatsoever. This result is achieved by discontinuing thedodging at some intermediate portion of the exposure. The total exposurewill be controlled as in conjunction with the preceding figures tocorrespond with a predetermined reference voltage 32 based upon thespeed of the emulsion of the positive material or photosensitivematerial employed. In this case, the integrating switch 30, subject toan additional reference voltage 36 serves to operate a switch 38 todiscontinue the effect of the A.C. inverse feedback amplifier after apredetermined quantity of light has been sensed by the phototube 28.Inasmuch as the reference voltages 32 and 36 are adjustable, the totalexposure can be varied in connection with Fig. 3 as well as Figs 1 and2, and the portion of the exposure during which dodging occurs can alsobe varied. Accordingly, with apparatus of the type shown in Fig. 3, thedegree of dodging can be varied infinitely between zero and one hundredpercent. Thus, with the apparatus and method involving Fig. 3 also,there is one component of exposing light varying over the photosensitivesurface as an inverse function of the sensed intensity and anothercomponent substantially equally illuminating the entire area of thetransparency.

Whereas only three forms of the invention have been described by way ofexample, the invention should not be restricted thereto beyond the scopeof the appended claims.

The apparatus disclosed herein has been claimed in application Ser. No.666,123, filed June 17, 1957, now Patent No. 2,960,019 of which this isa division.

I claim:

1. A photographic printing method comprising energizing a cathode raytube to direct a light spot of finite area through a transparency on aphotosensitive surface to be exposed, sensing the intensity of light towhich said surface is exposed, and during an exposing cycle controllingthe exposing light impinging on said surface to provide one exposinglight component varying over said surface as an inverse function of thesensed intensity, and another exposing light component substantiallyequally illuminating the entire area of said transparency.

2. A photographic printing method as set forth in claim 1 wherein saidcomponents of exposing light are provided simultaneously.

3. A photographic printing method as set forth in claim 1 wherein saidcomponents of exposing light are provided sequentially.

4. A photographic printing method as set forth in claim 1 wherein saidsubstantially equally illuminating component is produced optically bydiffusion of a portion of the light from said tube.

5. A photographic printing method as set forth in claim 1 wherein saidsubstantially equally illuminating component is produced electrically byrendering the control of said cathode ray tube independent of saidsensed intensity.

6. A photographic printing method as set forth in claim 1 wherein theproportions of said components are controlled by integrating light towhich said surface is exposed.

7. A photographic printing method as set forth in claim 1 wherein theexposing cycle is controlled by integrating light to which said surfaceis exposed.

. 8. A photographic printing method as set forth in claim 1 wherein theexposing cycle and proportions of said components arecontrolled byintegrating light to which said surface is exposed.

References Cited in the file of this patent UNITED STATES PATENTS2,757,571 Loughren Aug. 7, 1956

